Pin CI Ubuntu version to 22.04

Author: smheidrich

.github/workflows/test.yml

@@ -18,7 +18,7 @@ jobs:
     strategy:
       matrix:
         python-version: ["3.7"]
-        os: ["ubuntu-latest", "windows-latest", "macos-latest"]
+        os: ["ubuntu-22.04", "windows-latest", "macos-latest"]
         include:
           # set OS-specific cache paths
           - os: ubuntu-latest